Major Milestones In X-ray Astronomy

by WKTJune 6, 2002 ::
In September, 1949, a team led by Herbert Friedman of the Naval Research Laboratory detected weak X-ray emission from the solar corona, the hot outer layers of the Sun's atmosphere. Their experiment consisted of a collection of small Geiger counters aboard a captured German V-2 rocket. It took more than a decade before a greatly improved detector discovered X-rays coming from sources beyond the solar system.

In 1962, a team of scientists under the direction of Riccardo Giacconi at American Science and Engineering in Cambridge, MA., used a small
X-ray detector aboard an Aerobee rocket to discover Scorpius X-1, the first source of X-rays outside our solar system. Forty years later, over 100,000 X-ray sources have been detected, the most
distant of which is 13 billion light years from Earth. This extraordinary leap in sensitivity has been due, in large part, to the development of telescopes that can focus X-rays.

The first imaging X-ray telescope was made by Giacconi and collaborators. It was flown on a small sounding rocket in October 1963 and made crude images of hot spots in the upper atmosphere of the
sun. This telescope was about the same diameter and length as the optical telescope Galileo used in 1610. Over a period of 380 years, optical telescopes improved in sensitivity by 100 million times
from Galileo's telescope to the Hubble Space Telescope. Remarkably, the Chandra X-ray Observatory represents a comparable leap in sensitivity over Giacconi's 1963 telescope, yet it took only 36
years!

Date: December 1970 to March 1973
Vehicle/Mission: Uhuru X-ray satellite
Agency/Country: NASA
Instruments/Detectors: Proportional counters (2 -20 keV)
Mirror Description: No mirrors
Highlights: Uhuru, the first satellite dedicated to the observation of cosmic X-ray sources, was equipped with a sensitive proportional counter attached to a viewing pipe to
locate the sources. It expanded the number of known sources to more than 400, showed that X-ray stars are neutron stars or black holes accreting matter from companions in binary star systems, and
discovered X-rays from hot gas in galaxy clusters.

Date: November 1978 to April 1981
Vehicle/Mission: Einstein X-ray Observatory
Agency/Country: NASA
Instruments/Detectors: Proportional counters/microchannel plate imager/solid state spectrometer/crystal spectrometer (0.2 -4 keV)
Mirror Description: 4 nested mirror pairs with an area of 350 sq cm and a resolution of 3-5 arc sec.
Highlights: The first large X-ray telescope with mirrors. Einstein made the first X-ray images of shock waves in supernova remnants, hot gas in galaxies and clusters of
galaxies. Einstein also located accurately over 7000 X-ray sources, including stellar coronas, X-ray binaries, galaxies and quasars. It made possible a new way to study the mysterious dark matter
that is present in galaxies and clusters of galaxies and showed that most of the X-ray background is probably due to discrete sources.

Date: June 1990 to February 1999
Vehicle/Mission: Roentgensatellite (ROSAT)
Agency/Country: Germany
Instruments/Detectors: Proportional counters/microchannel plate imager (0.1 -2.5 keV)
Mirror Description: 4 nested mirror pairs with 1140 sq cm area and 3 arc sec resolution Highlights: ROSAT expanded the number of known X-ray
sources to 125,000 and proved to be especially valuable for investigating the multi-million degree gas present in the upper atmospheres of many stars, made the first detection of radiation from the
hot surface of a neutron star, and comets. It resolved 70% of soft X-ray background, and made an extensive catalogue of galaxy clusters.

ASCA

Date: 1993 to July 2000
Vehicle/Mission: Advanced Satellite for Cosmology and Astrophysics (ASCA)
Agency/Country: Japan
Instruments/Detectors: Proportional counters/CCDs (0.4 -12 keV)
Mirror Description: 120 nested foil mirrors with 1300 sq cm area and resolution of 180 arc sec
Highlights: The ASCA X-ray observatory with low resolution mirrors, and the first large CCD detectors, was especially designed to study the detailed spectra of X-ray sources
in supernova remnants, active galactic nuclei and galaxy clusters. Found first evidence of the gravitational redshift due to the strong gravitational field around a black hole.

RXTE

Date: December 1995 to present
Vehicle/Mission: Rossi X-ray Timing Explorer (RXTE)
Agency/Country: NASA
Instruments/Detectors: Proportional counters (2 -50 keV) and scintillation counters (15 -200 keV)
Mirror Description: No mirrors
Highlights: Although RXTE does not have focusing X-ray mirrors, it has the unique capability to study rapid time variability in the emission of cosmic X-ray sources over a
wide band of X-ray energies, and has made valuable contributions to our understanding of the behavior of matter in the vicinity of neutron stars and black holes. Extremely rapid variability observed
by RXTE may be evidence for the dragging of space by strong gravity in the vicinity of black holes.

BeppoSAX

Date: April 1996 to April 2002
Vehicle/Mission: BeppoSAX
Agency/Country: Italy & The Netherlands
Instruments/Detectors: Proportional counters (0.1 -10 keV), scintillation counters (3 -120 keV) and phoswich detectors (15 -300 keV)
Mirror Description: No mirrors
Highlights: BeppoSAX is the first X-ray mission with a scientific payload covering more than three decades of energy - from 0.1 to 300 keV, with moderate imaging capability.
BeppoSAX has proved to be especially useful for wide band spectra of active galactic nuclei, and for studying gamma-ray bursts by determining their positions with an unprecedented precision and
monitoring their X-ray afterglow.

Chandra X-ray Observatory

Date: July 1999 to present
Vehicle/Mission: Chandra X-ray Observatory
Agency/Country: NASA
Instruments/Detectors: High Resolution Imager/CCDs, grating spectrometers (0.1 -10 keV)
Mirror Description: 4 nested pairs with 1145 sq cm area and 0.5 arc sec resolution.
Highlights: Chandra's unique sensitivity and precision have made possible significant advances in many areas of astronomy. Of
particular importance are discoveries relating to the life cycles of stars, the number and nature of black holes in the universe, the generation by neutron stars of high energy matter and anti-matter
particles, and the formation and evolution of galaxies. Chandra has traced the distribution of elements such as oxygen, neon, silicon and iron in
supernova remnants, and made spectacular images of the pulsar wind nebulas in the Crab Nebula and Vela supernova
remnant.

Chandra observations have revealed the presence of many previously undetected stellar black holes in nearby galaxies, including a possible new class of intermediate mass black holes. Chandra's detailed spectral studies of the activity of supermassive black holes in the centers of galaxies have detected winds
coming from the centers of galaxies, and X-ray jets hundreds of thousands of light years in length that can be traced back to the central supermassive black
holes. Very long Chandra exposures have shown that there may be twice as many active supermassive black holes in the universe as previously thought. Chandra is enabling astronomers to study how
clusters of galaxies are formed and how they change with time, and to investigate the nature of the dark matter that comprises most of the mass of the universe.

XMM-Newton

Date: December 1999 to present
Vehicle/Mission: XMM-Newton
Agency/Country: European Space Agency
Instruments/Detectors: CCD cameras and spectrometers (0.1 - 12 keV)
Mirror Description: 3 modules each with 58 wafer-thin mirrors for a total area of 4300 sq cm and a resolution of 5 arc sec
Highlights: With its large collecting area, spectrometers and moderate resolution, XMM-Newton is especially designed to make detailed studies of the spectra of supernova
remnants, accretion disks around black holes, stars and other types of sources. Notable achievements have been to accurately measure the energy budget of radiation from a black-hole accretion disk,
the detection of heavy elements in the spectrum of the afterglow of a gamma-ray burst, the mapping of elements in supernova remnants, and detailed spectra of active galactic nuclei.